A known spring installation structure in a seat reclining device is disclosed in Japanese Patent Laid-Open Publication No. 11-056513. The disclosed seat reclining deice includes a lower arm fixed to a seat cushion, an upper arm fixed to a seat back and rotatably engaging with the lower arm, and a locking mechanism restricting the rotation of the upper arm and the lower arm. A spring for normally biasing the locking mechanism in a direction in which the rotation of the lower arm and the upper arm is restricted is disposed in a hole formed at a center portion of the lower arm. One end of the spring engages with an outer surface while the other end of the spring engages with a cam of the locking mechanism.
Further, another known spring installation structure is disclosed in U.S. Pat. No. 6,328,383. The disclosed device has a similar structure to the disclosed device in Japanese Patent Laid-Open Publication No. 11-056513. The spring biasing the locking mechanism in the direction in which the rotation of the lower arm and the upper arm is restricted is disposed in a detent formed at the center portion of the lower arm. One end of the spring engages with an inner portion of the detent while the other end engages with a rotating shaft to which an operating lever is connected.
According to the above-mentioned structures, the spring is disposed beyond a surface in the width direction of the lower arm, or an extending portion is formed in the width direction of the lower arm in order to obtain a space for accommodating therein the spring. Thus, a width of the seat reclining device becomes larger. It is desirable to obtain a space in the width direction of the seat as large as possible for providing a comfortable seating space for a passenger in a limited space of the vehicle. However, according to the above-mentioned devices, a passenger may feel uncomfortable when sitting in the seat equipped with such the reclining devices each having a large width.
Thus, a need exists for a seat reclining device having a small dimension in a width direction.